1. Field of the Invention
This invention relates to an improved method for estimating seismic velocities used in generating seismic sections. More particularly, it pertains to a method for improving the signal to noise ratio of a stacked trace of common midpoint seismic data by accounting for the effects of azimuthal anisotropy.
2. Description of the Prior Art
Seismic prospecting methods record the reflection time of seismic signals imparted into the earth. The determination of the reflection depth, which is of ultimate importance to the prospector, depends on the correct estimation of the subsurface seismic velocities. One of the methods used to estimate this unknown velocity is to measure the difference in the time of receipt of a particular seismic reflection between one geophone and another. To the extent that reflection time predictably increases with distance between the source and each successive receiver, one has a good estimate of the subsurface velocity.
For approximately a half of century, the processing and interpretation of compressional wave data has proceded under the assumption of azimuthal isotropy, i.e., the velocity of compressional waves is assumed to be independent of the azimuth of the vertical plane containing the source and receiver. Using this assumption, it is well known that the relation between travel time and offset (the normal moveout) for a reflector recorded over an approximately horizontally layered earth by a collection of common midpoint traces is given by the simple formula: ##EQU1## where: T.sub.o =zero-offset travel time;
T=offset-dependent travel time; PA1 r=source-receiver offset; PA1 v=normal moveout velocity.
Note that the travel time is independent of trace azimuth.
During the past decade, however, three-dimensional (3D) seismology has become a routine technique for helping to solve problems of exploration and exploitation geophysics (Nestvold, 1992). While the common midpoint technique (Mayne, 1962) dictated the basic geometry of 2D seismic acquisition for three decades, the collection of 3D data is now accomplished using a variety of different approaches.
The very nature of 3D seismic data acquisition results in the data being collected along a variety of source receiver azimuths. Many 3D data acquisition techniques are relatively straightforward generalizations of well known 2D methods; examples include multi-streamer marine operations and in-line swath shooting on land. For these quasi-2D methods, traces in a common midpoint tend to be restricted to a narrow range of source-receiver azimuths and the offsets of the traces are strongly correlated with trace azimuths.
More advanced 3D data acquisition techniques (e.g., Crews, et al., 1989 and Musser, et al., 1989) provide data which contain a rich assortment of trace azimuths in a common midpoint. Furthermore, these azimuths are not strongly correlated with the trace offsets.
If the subsurface exhibits a significant degree of azimuthal anisotropy, conventional isotropic techniques for performing velocity analysis and normal moveout correction may fail to adequately account for the travel time behavior of recorded seismic data.
2D-data and 3D-data collected in a quasi-2D fashion may not be suitable for advanced analysis for the effects of azimuthal anisotropy because of the restricted range of trace azimuths available in a common midpoint gather. On the other hand, the collection of 3D data using the methods similar to those described in Crews and/or Musser, containing rich distributions of azimuths, can lead to greatly improved subsurface images and facilities a fundamentally new way of analyzing reflection seismic data for the effects of azimuthal anisotropy.